modified ATX (current mode)

Thread Starter


Joined Feb 24, 2008
The attached file is a modified schematic of ATX power supply which I want to use as current mode supply. So the output current from current sense transformer was used at Pin4 (Deadtime Control) of IC TL494, through variable resistor to adjust the desired current (9A). Please comment if this technique can deliver a constant current output. Voltage regulation is not necessary.




Joined Jul 17, 2007
I suppose it could be used as a constant current source, if you had a sense resistor in the return current path, and use the voltage across it to control the dead time via pin 4.
If you used a 0.05 Ohm resistor, you'd get 0.45v across it at 9A, and dissipate 4.05W power. You could use a 10W non-inductive resistor. Use an opamp as a noninverting amplifier with adjustable gain to set the voltage on pin 4 to maintain 9A flow.

You'd obviously need a circuit to pull pin 4 to 2.8v if the output voltage exceeded safe limits. That might be as simple as adding a 1k resistor between the output of the op amp and pin 4, and a Zener diode of an appropriate voltage from the supply output to pin 4; suitable Zener being rated (max_safe_output_voltage - 2.8).

However, you would really want to ensure that you didn't get close to the voltage rating of the capacitors/diodes/transistors, or you'd burn things up. For electrolytic caps, max voltage applied should be 1/2 of their rating. You also don't want to attempt to exceed the power rating of the supply.

Looks like when pin 4 is at 0v, duty cycle is near 47%, when @ 2.8v duty cycle is zero, and it's quite linear.